In the documentary Lily also opens up on her battle with bulimia, revealing: ‘I used to vomit after meals. It’s not something I’m proud of.

‘I thought I looked good and it was great to be able to try on clothes and feel a million dollars. But I wasn’t happy, I really wasn’t.’

Lily Allen: From Riches To Rags focuses on the launch of fashion range Lucy In Disguise (Channel 4)

Lily and Sarah also lifted the lid on their rocky relationship in the documentary, with Lily admitting: ‘Ninety per cent of the time we ended up rolling about on the floor, pulling each other’s hair out.’

Sarah added: ‘She gave me two black eyes once – that was quite a low point.’

But the sisters have now made their peace, and Lucy In Disguise is thriving – with stars including Jessie J venturing into the London vintage store.

Last week the Do It Like A Dude singer used her Twitter account to declare her love for vintage clothing, prompting Lily to respond: ‘Come to my shop then! Lucyindisguise. I believe the orange moschino jacket you wear in dude vid, was one of ours.’

The first part of the documentary airs next Tuesday at 10pm.

Lily and Sarah Allen talk about their relationship on Rags to Riches (Picture: Channel 4)